A wall of a cardboard box or container generally includes a layer of corrugating medium positioned between thin sheets of linerboard, which form the outer plies. The corrugating medium forms the wavy center layer of the wall and may be used, for example, to cushion and/or protect item(s) inside the cardboard box or container. Corrugating medium is generally made from high yield hardwood pulps blended with recycled fiber such as old corrugating containers (OCC) or double-lined kraft clippings (DLK). Corrugating medium may also be produced from 100% recycled fiber furnish and/or post-consumer recycled fiber content without reducing its ability to protect an item(s) stored within the corrugating box or container.
The high yield hardwood pulps used in manufacturing corrugating medium may be produced using semichemical pulping processes including caustic carbonate pulping, neutral sulfite semichemical pulping (NSSC), and green liquor pulping. These existing processes initially use a liquor to cook the woodchips in a substantial amount of alkali to facilitate partial delignification and to minimize carbohydrate degradation. This is considered important or necessary for a corrugating medium manufactured from the pulp to possess desirable physical properties.
Accordingly, during the initial cooking stage of existing pulping processes, woodchips are placed into a digester(s) including a basic solution of alkali-containing cooking liquor. The weight percent of alkali (e.g., NaOH, Na2CO3, Na2SO3, NaHCO3, K2CO3, KHCO3, NH4OH) on a bone dry wood basis generally ranges from about 4% to about 8% expressed as alkaline oxide (e.g., Na2O). Bone dry wood is defined as moisture-free wood. The yield (the ratio of product output to raw material input) using these existing pulping processes generally ranges from about 70% to about 85%. The resultant pulp is then fiberized, pressed, and washed, thereby separating liquid filtrates (e.g., weak liquor) and solid filtrates from the pulp so that the pulp may be further refined. During the final refining stages, about 25% to about 50% recycled fiber is added to the pulp. The pulp is then formed into corrugating medium by a papermachine. The liquid filtrates separated from the pulp are evaporated, and the solid filtrates are burned in recovery boilers or fluidized bed reactors.
Vast amounts of capital, labor, and energy are generally expended to recover energy and chemicals associated with the significant amounts of alkali used during existing pulping processes. For example, it is desirable for the bulk of the alkali used during the initial cooking stage to be recovered from the liquid filtrates during a chemical recovery process and recycled back to the digester(s). The chemical recovery process generally includes evaporating excess water from the liquid filtrates to maximize the concentration of the recovered alkali, which requires significant amounts of energy. Furthermore, using large amounts of alkali may have detrimental effects on the environment.
It would be desirable to have a pulping process that assists in addressing one or more of the above disadvantages.